1. Field of the Invention
The present invention relates to the control of a TDM bus and, more particularly, to a system for controlling line cards in a telecommunications terminal for multiplexing and demultiplexing subscriber line information to and from a TDM bus.
2. Description of the Prior Art
In telephony, time division multiplexing (TDM) is used as a method for transmitting information from several subscriber lines or ports over a single telephone transmission line. The information for each subscriber is multiplexed onto and demultiplexed from time slots or channels on the TDM bus. Thus, a time slot or channel on the TDM bus must be provided for each subscriber line serviced by the TDM bus. As a practical matter, only a limited number of time slots are available on a TDM bus, as the time slots must be repeated periodically in order to convey the subscriber line information without interruption. Thus, TDM buses transmit information in repeated frames comprising a fixed number of time slots or channels.
Heretofore, in most cases a fixed time slot was assigned to each subscriber line for transmitting and receiving subscriber line information. A fixed timing signal in the form of a strobe was supplied to each line card serving a subscriber line, so that information from the subscriber line would be multiplexed onto or demultiplexed from the TDM bus only during the fixed, assigned time slot.
An alternative method used previously was to have timing logic on each card that could programmably deliver the channel in an assigned time slot with respect to a reference synchronization pulse. This required a communication channel to each card and some method so that each card can identify a message unique to it. Clearly, this method was cumbersome and complex.
The prior art systems did not provide a convenient method for the reassignment of time slots and therefore the systems were rather rigid and inflexible. Concentration could not be easily accommodated, since a time slot was fixedly assigned to each subscriber line. Advantage could not be taken of the fact that not all subscriber lines are used simultaneously. In addition, the prior art systems could not service subscriber lines requiring the transmission of high-speed digital information beyond the bandwidth capability of an individual time slot. Thus, higher speed telephone transmission lines had to be dedicated to subscribers requiring the transmission of high-speed digital information.
The provision of individual strobe signals to each of the line circuits created a wiring nightmare, in that separate strobe lines had to be extended to each of the line cards, resulting in large wiring harnesses and timing irregularities that can result from excessively long strobe lines.